Scheduled for launch in January 2024, the PACE mission represents NASA’s next investment in ocean biology, clouds, and aerosol data records. A key feature of PACE is the inclusion of an advanced satellite radiometer known as the Ocean Color Instrument (OCI), a global mapping radiometer that combines multispectral and hyperspectral remote sensing. A critical requirement for OCI is the high-contrast or spatial crosstalk specification (also referred to as in-field stray-light response). The requirement states that for global top-of-atmosphere radiances based on measured MODIS radiances, the global average residual contamination shall be less than 0.4% for 350 nm, 360 nm, 385 nm, 555 nm, 583 nm, 820 nm and 865 nm and less than 0.20% for all other multispectral bands. Accurate resolution of high contrast in TOA radiance images is important to estimate stray light contamination due to clouds, for studying small scale features like ocean fronts and for working in coastal and estuarine areas where the scales are 1km. This occurs in all wavelengths in the spatial direction. Knowledge of high contrast resolution makes up part of the artifact budget. Accurate measurement of the high-contrast performance of OCI requires laboratory Ground Support Equipment (GSE) that projects a scene of sufficient quality that the unwanted stray light of the GSE itself is not confused with the stray light response of the telescope. This paper concerns the development, analyses and test of the GSE to ensure the quality of the projected image is sufficient to verify the OCI requirements. Optical models were developed for both the instrument as well as the GSE and laboratory environment. Simulation of various non-ideal parameters were critical to accurately predict performance. Measurements using COTS cameras and lenses were also made of the projected GSE image to reasonably verify the optical model predictions. Measured and modelled results from OCI are discussed.
The Ocean Color Instrument (OCI) is a sensor on the upcoming Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission, scheduled for launch in early 2024. OCI is a grating spectrometer with hyperspectral coverage from the ultraviolet (about 310 nm) to near-infrared (about 900 nm), with additional filtered channels in the short-wave infrared (940 nm – 2260 nm). This instrument will provide ocean color science data to continue the data sets collected by heritage sensors MODIS, SeaWiFs, and VIIRS, but with increased spectral coverage and improved accuracy. In order to achieve the high levels of accuracy demanded by the science community, a rigorous ground test program was conducted to calibrate the instrument and ensure that the calibration can be transferred to on-orbit operations. Some calibration parameters can only be measured during pre-launch testing; one such parameter is the polarization sensitivity. Polarization testing measured the Mueller matrix components needed to determine the polarization sensitivity for all spectral bands for a series of telescope scan angles covering the expected on-orbit scan range. Results indicate that the sensitivity is below 0.6 % except at the shortest wavelengths (less than 340 nm) and was characterized to better than 0.1 % above 340 nm. This indicates that any polarized scenes measured on orbit can be corrected for with a high degree of confidence.
The Ocean Color Instrument (OCI) is the primary instrument on NASA’s Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission. OCI flight model will be a hyper-spectral scanning (HSS) radiometer designed to measure spectral radiances from the ultraviolet to shortwave infrared (SWIR) currently in development at the Goddard Space Flight Center (GSFC). The OCI engineering model provides hyperspectral coverage from 600nm to 885nm and 7 discrete spectral bands from 940nm to 2260nm. The engineering model’s radiometric response and sensitivity to polarized light has been measured as a function of scan angle in ambient, and in thermal vacuum for nadir viewing. This paper will state the polarization requirements, describe the various polarization measurements and present and discuss the polarization measurement results.
The Bidirectional Reflectance Distribution Function (BRDF) and Total Hemispherical Reflectance (THR) of two candidate black diffuse materials for the dim calibration targets of the NASA GSFC PACE Ocean Color Instrument (OCI) were reported in the SPIE conference last year. In this paper, we present new BRDF and THR results of the two black diffuse materials following additional UV exposure and solar wind tests. The BRDF measurements for five samples of each two black diffuse material were made at incident angles of 0° and 45° and at the wavelengths of 360 nm, 600 nm, and 1600 using the Table-top Goniometer (TTG) located in the Diffuser Calibration Laboratory (DCL) at NASA GSFC. The THR of the samples, 15 mm in diameter, was measured using a commercial UV-VIS-NIR spectrophotometer from 200 nm to 2500 nm. The spectral THR results of the two black diffuse materials exposed to UV and solar wind show an approximate 10 % higher reflectivity than the unexposed samples. The spectral profiles of the THR of the exposed and unexposed samples are relatively similar. The BRDF results at the incident angle of 45° show different trends in the forward and backward scattering regions, while those at normal incident angle are consistent with the THR results. We will also present the details of the samples’ surface features and the comparison of the 0°/45° BRDF and THR results, demonstrate the significance of background subtraction in the THR measurements for small, low reflectance samples, and discuss validation of BRDF scale, measurement repeatability, and major contributions of uncertainty.
The Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission will launch no earlier than summer 2022. The primary payload is the Ocean Color Instrument (OCI). OCI is a hyperspectral imaging radiometer that will measure top-ofatmosphere radiances from 340nm to 2260nm at approximately 1km spatial resolution. The spectral resolution will be 5nm from 340nm to 890nm to enable the production of innovative ocean color products on a global scale (OCI will provide global coverage every 2 days). There are 7 different multispectral bands in the shortwave infrared to support atmospheric correction for ocean color and aerosol and cloud studies. Ocean color applications require state of the art radiometric accuracy (approximately 0.5%, excluding the absolute calibration uncertainty). Considerable effort has been invested in the planning of the prelaunch calibration campaign and the on-orbit calibration capabilities. This paper describes the current plans for the prelaunch calibration and characterization campaign of the OCI Engineering Test Unit (ETU), which is scheduled to begin towards the end of 2019. The prelaunch calibration campaign will characterize all sensor characteristics that are expected to influence radiometric sensitivity: absolute calibration (i.e. radiometric gains), signal to noise ratio, nonlinearity, response versus scan angle, dynamic range, signal to noise ratio, and sensitivities to polarization and temperature. In addition to these one-time characterization tests, two types of tests have been developed that monitor the evolution of several OCI radiometric characteristics: a Limited Performance Test (LPT, expected duration about 8 hours), and a Comprehensive Performance Test (CPT, expected duration about 2 days).
We report the Bidirectional Reflection Distribution Function (BRDF) and Total Hemispheric Reflectance (THR) results of several low reflectance materials using a Table-top Goniometer (TTG) and a commercial UV-VIS-NIR spectrophotometer in support of the NASA GSFC PACE project. The newly developed TTG was utilized to perform the BRDF measurements for several black candidate samples in in-plane and out-of-plane configurations from 300 nm to 2000 nm. These measurements demonstrated the BRDF capability of the TTG to calibrate the dim calibration target with a reflectance of approximately 2 % for the OCI of the PACE project. The spectral THR of the black samples from 200 nm to 2500 nm was determined using a 10 % reflectance diffuse black standard and a monochromator-based light source equipped with a 150 mm diameter integrating sphere. The THR measurement is used to compliment and validate the BRDF measurements acquired from these samples. In this presentation, we also show examples of UV induced BRDF and THR changes on two black coatings. We will discuss validation of the BRDF scale, source stability, measurement repeatability, instrument signature, and uncertainty components.
The Advanced Topographic Laser Altimeter System (ATLAS) will be the only instrument on the Ice, Cloud, and Land
Elevation Satellite -2 (ICESat-2). ICESat-2 is the 2nd-generation of the orbiting laser altimeter ICESat, which will
continue polar ice topography measurements with improved precision laser-ranging techniques. In contrast to the
original ICESat design, ICESat-2 will use a micro-pulse, multi-beam approach that provides dense cross-track sampling
to help scientists determine a surface's slope with each pass of the satellite. The ATLAS laser will emit visible, green
laser pulses at a wavelength of 532 nm and a rate of 10 kHz and will be split into 6 beams. A set of six identical,
thermally tuned optical filter assemblies (OFA) will be used to remove background solar radiation from the collected
signal while transmitting the laser light to the detectors. A seventh assembly will be used to monitor the laser center
wavelength during the mission. In this paper, we present the design and optical performance measurements of the
ATLAS OFA in air and in vacuum prior to their integration on the ATLAS instrument.
Risk mitigation activities associated with a prototype imaging Fabry-Perot Interferometer (FPI) system are continuing at
the NASA Langley Research Center. The system concept and technology center about enabling and improving future
space-based atmospheric composition missions, with a current focus on observing tropospheric ozone around 9.6
micron, while having applicability toward measurement in different spectral regions and other applications. Recent
activities have focused on improving an optical element control subsystem to enable precise and accurate positioning
and control of etalon plates; this is needed to provide high system spectral fidelity critical for enabling the required
ability to spectrally-resolve atmospheric line structure. The latest results pertaining to methodology enhancements,
system implementation, and laboratory characterization testing are discussed.
In this paper, PMN-PT single crystal piezoelectric stack actuators and flextensional actuators were designed, prototyped and characterized for space optics applications. Single crystal stack actuators with footprint of 10 mm x 10 mm and the height of 50 mm were assembled using 10 mm x 10 mm x 0.15 mm PMN-PT plates. These actuators showed stroke of 65 - 85 μm at 150 V at room temperature, and > 30 μm stroke at 77 K. Flextensional actuators with dimension of 10 mm x 5 mm x 7.6 mm showed stroke of > 50 μm at room temperature at driving voltage of 150 V. A flextensional stack actuator with dimension of 10 mm x 5 mm x 47 mm showed stroke of ~ 285 μm at 150 V at room temperature, and > 100 μm at 77K under driving of 150 V should be expected. The large cryogenic stroke and high precision of these actuators are promising for cryogenic optics applications.
Risk mitigation activities for a prototype imaging Fabry-Perot Interferometer (FPI) system, development originating
within NASA's Instrument Incubator Program (IIP) for enabling future space-based atmospheric composition missions,
are continuing at NASA Langley Research Center. The system concept and technology are focused on observing
tropospheric ozone around 9.6 micron, but also have applicability toward measurement of other trace species in different
spectral regions and other applications. The latest results from performance improvement and laboratory
characterization activities will be reported, with an emphasis placed on testing performed to evaluate system-level
radiometric, spatial, and spectral measurement fidelity.
An airborne imaging Fabry-Perot Interferometer (FPI) system was developed within NASA's Instrument Incubator
Program (IIP) to mitigate risk associated with implementation of such a device in future space-based atmospheric
remote sensing missions. This system is focused on observing tropospheric ozone through measuring a narrow
spectral interval within the strong 9.6 micron infrared ozone band at high spectral resolution, while the concept and
technology also have applicability toward measurement of other trace species and other applications. The latest
results from laboratory testing and characterization of enabling subsystems and the overall instrument system will
be reported, with an emphasis placed on testing performed to evaluate system-level radiometric, spatial, and spectral
measurement fidelity.
The Tropospheric Trace Species Sensing Fabry-Perot Interferometer (TTSS-FPI) was a NASA Instrument Incubator
Program (IIP) project for risk mitigation of enabling concepts and technology applicable to future NASA Science
Mission Directorate atmospheric chemistry measurements. Within IIP an airborne sensor was developed and
laboratory-tested to demonstrate the instrument concept and enabling technologies that are also applicable to the
desired geostationary-based implementation. The concept is centered about an imaging Fabry-Perot interferometer
(FPI) observing a narrow spectral interval within the strong 9.6 micron ozone infrared band with a spectral
resolution ~0.07 cm-1, and also has applicability to and could simplify designs associated with sensors targeting
measurement of other trace species. Results of testing and characterization of enabling subsystems and the overall
instrument system are reported; emphasis is placed on recent laboratory testing performed to evaluate system-level
radiometric, spatial, and spectral measurement fidelity.
We propose an algorithm for recovering the overlapped harmonic components of a multiplex Fabry-Pérot interferometer (MFPI) spectrum. A potential solution that utilizes the multiresolution properties of the MFPI spectrum is presented. The corrupted harmonic components are detected by comparing consecutive harmonic pairs at the same sampling rate and window length. The overlapped spectral components are adjusted with correction terms that have been determined from the detection stage.
High-finesse Fabry-Perot interferometers are useful tools when it comes to high-resolution spectroscopy and narrow-band filtering. Rugged, reliable, narrow-band tunable filters are of special interest for remote sensing from ground, airborne and space-based platforms. This report discusses the results of a numerical analysis and experimental study of such a filter with its design based on the unique features of the photonic crystal interferometer (PCI). An optimal choice of the PCI components and their parameters allows achieving a sub-angstrom bandpass combined with a broad tunability range (< 10 nm) in the visible spectral regions. The prototype PCI system has the tunability over a 1 nm with a bandpass near 0.02 nm and an acceptance angle about 1 angular degree. Detailed consideration is given to the imaging characteristics of such an interferometer, and their dependence upon the quality of the individual components (i.e. the mirror substrates, dielectric layers, etc.). In summary, we present the results of the theoretical analysis and experimental study of the spectral and imaging characteristics of a high-finesse PCI, and their impairments due to deviations from the required parameters of the optical elements.
The Tropospheric Trace Species Sounder is a spatially imaging, spectrally tunable airborne sensor focused on demonstrating a new capability to make important measurements of tropospheric ozone. The sensor system is based upon a cryogenically cooled dual etalon infrared Fabry-Perot interferometer. The instrument package is designed to operate autonomously on a high altitude aircraft platform. We present herein details of the airborne instrument's hardware and component test results.
Monitoring tropospheric chemistry from space is the next frontier for advancing present-day remote sensing capabilities to meet future high-priority atmospheric science measurement needs. The Tropospheric Trace Species Sensing Fabry-Perot Interferometer (TTSS-FPI) is a NASA Instrument Incubator Program (IIP) project for risk mitigation of enabling concepts and technology applicable to future Office of Earth Science (OES) atmospheric chemistry measurements. While the intended implementation for future science missions is a geostationary based measurement of tropospheric ozone and other trace species, a multispectral imaging airborne sensor system is being developed within IIP to demonstrate the instrument concept and enabling technologies that are also applicable to space-based configurations. The concept is centered about an imaging Fabry-Perot interferometer (FPI) observing a narrow spectral interval within the strong 9.6 micron ozone infrared band with a spectral resolution ~ 0.07 cm-1. This concept is also applicable to and could simplify designs associated with atmospheric chemistry sensors targeting other trace species (which typically require spectral resolutions in the range of 0.01 - 0.1 cm-1), since such an FPI approach could be implemented for those spectral bands requiring the highest spectral resolution and thus simplify overall design complexity. An overview of this IIP project addressing the measurement and instrument concepts, enabling technologies, approach for development and demonstration, and a summary of progress-to-date will all be reported. This will include sensor radiometric, spectral, and spatial characterization activities relevant to measurement concept validation. Subsequent manuscripts following in these proceedings will focus on the airborne prototype system under development and a corresponding spaceflight concept study, respectively.
We present results of studies of instrument concepts for a spaceborne imaging Fabry-Perot interferometer to measure tropospheric ozone. Ozone is recognized as one of the most important trace constituents of the troposphere. Tropospheric ozone is responsible for acute and chronic human health problems and contributes toward destruction of plant and animal populations. Furthermore, it is a greenhouse gas and contributes toward radiative forcing and climate change. Tropospheric ozone levels have been increasing and will continue to do so as concentrations of precursor gases (oxides of nitrogen, methane, and other hydrocarbons) necessary for the photochemical formation of tropospheric ozone continue to rise. Space-based detection and monitoring of tropospheric ozone is critical for enhancing scientific understanding of creation and transport of this important trace gas and for providing data needed to help develop strategies for mitigating impacts of exposure to elevated concentrations of tropospheric ozone. Measurement concept details are discussed in a companion paper by Larar et al. Development of an airborne prototype instrument for this application is discussed by Cook et al. in another companion paper.
This paper presents the results of the analysis and experimental characterization of a narrow bandpass optical filter based on the Fabry-Perot interferometer configuration with a variable spacing between the mirrors allowing for a relatively wide spectral tunability. Such a filter with a high-throughput bandpass and sufficiently large aperture and acceptance angle is of practical interest for a high-resolution spectral measurements and remote sensing in the visible and infrared spectral regions. The Fabry-Perot filter (FPF) can be designed in a compact single-assembly architecture that can be accommodated within existing instruments and should provide a stable performance under variable thermal and mechanical conditions, including space and airborne platforms. Possible applications of the filter include high-resolution multi-spectral imaging, terrain mapping, atmosphere and surface parameters measurements, and detection of chemical and biological agents.
Though a Phase I and II NASA SBIR initiative Michigan Aerospace Corp. has demonstrated a spaceflight qualified, tunable infrared Fabry Perot etalon. The design included use of single crystal ferroelectric actuators for tuning the etalon gap. The operational wavelength range of this etalon was designed for 10-14μm and utilized Zinc-Selenide for the plate substrate with a plate reflectivity of 0.8. At a temperature of 193 K and 0.4 milli-torr, the etalon achieved a finesse of 11.8, a bandpass of 1.23 nm and had a free-spectral range of 14.2 nm at the test wavelength of 10.013 μm. At this temperature, the etalon was tunable over ~8.5 free-spectral ranges, or 45 μm in gap spacing. Testing concluded that the etalon will retain 57% of its total dynamic range at a temperature of 75 K compared to its dynamic range at room temperature. The design was qualified for a Delta-II launch vehicle vibration specification.
An instrument concept for an Imaging Multi-Order Fabry-Perot Spectrometer (IMOFPS) has been developed for measuring tropospheric carbon monoxide (CO) from space. The concept is based upon a correlation technique similar in nature to multi-order Fabry-Perot (FP) interferometer or gas filter radiometer techniques, which simultaneously measure atmospheric emission from several infrared vibration-rotation lines of CO. Correlation techniques provide a multiplex advantage for increased throughput, high spectral resolution and selectivity necessary for profiling tropospheric CO. Use of unconventional multilayer interference filter designs leads to improvement in CO spectral line correlation compared with the traditional FP multi-order technique, approaching the theoretical performance of gas filter correlation radiometry. In this implementation, however, the gas cell is replaced with a simple, robust solid interference filter. In addition to measuring CO, the correlation filter technique can be applied to measurements of other important gases such as carbon dioxide, nitrous oxide and methane. Imaging the scene onto a 2-D detector array enables a limited range of
spectral sampling owing to the field-angle dependence of the filter transmission function. An innovative anamorphic optical system provides a relatively large instrument field-of-view for imaging along the orthogonal direction across the detector array. An important advantage of the IMOFPS concept is that it is a small, low mass and high spectral resolution spectrometer having no moving parts. A small, correlation spectrometer like IMOFPS would be well suited for global observations of CO2, CO, and CH4 from low Earth or regional observations from Geostationary orbit. A prototype instrument is in development for flight demonstration on an airborne platform with potential applications to atmospheric chemistry, wild fire and biomass burning, and chemical dispersion monitoring.
Space-based observation of tropospheric trace species has been identified as a high-priority atmospheric science goal. In particular, global and regional measurements of lower atmosphere ozone concentrations are critical to both enhancing scientific understanding and to expanding capabilities for pollution monitoring. The interferometer addressed here will be a spatially imaging, spectrally tunable airborne sensor focused on making such important tropospheric ozone measurements, and is designed to be a risk-reduction and proof-of-concept test-bed for developing the corresponding orbiting instrument also based upon a dual etalon Fabry-Perot interferometer. We present herein details of the airborne
instrument design and development process, including parameter specifications for the interferometer and other enabling subsystems, as well as plans for integration, test, and characterization in the laboratory.
Monitoring tropospheric chemistry from space is the next frontier for advancing present-day remote sensing capabilities to meet future high-priority atmospheric science measurement needs. Paramount to these measurement requirements is that for tropospheric ozone, one of the most important gas-phase trace constituents in the lower atmosphere. Such space-based observations of tropospheric trace species are challenged by the need for sufficient horizontal resolution to identify constituent spatial distribution inhomogeneities (that result from non-uniform sources/sinks and atmospheric transport) and the need for adequate temporal resolution to resolve daytime and diurnal variations. Both of these requirements can be fulfilled from a geostationary Earth orbit (GEO) measurement system. The Tropospheric Trace Species Sensing Fabry-Perot Interferometer (TTSS-FPI) was recently selected for funding within NASA’s Instrument Incubator Program (IIP). Within this project we will develop and demonstrate a multispectral imaging airborne system to mitigate risk associated with an advanced atmospheric remote sensor intended for geostationary based measurement of tropospheric ozone and other trace species. The concept is centered about an imaging Fabry-Perot interferometer (FPI) observing a narrow spectral interval within the strong 9.6 micron ozone infrared band with a spectral resolution ~0.07 cm-1. This concept is also applicable to and could simplify designs associated with atmospheric chemistry sensors targeting other trace species (which typically require spectral resolutions in the range of 0.01 - 0.1 cm-1), since such an FPI approach could be implemented for those spectral bands requiring the highest spectral resolution and thus simplify overall design complexity. The measurement and instrument concepts, approach for development and demonstration within IIP, and a summary of progress-to-date will all be reported.
Monitoring tropospheric chemistry from space is the next frontier for advancing present-day remote sensing capabilities to meet future high-priority atmospheric science measurement needs. Paramount to these measurement requirements is that for tropospheric ozone, one of the most important gas-phase trace constituents in the lower atmosphere. Such space-based observations of tropospheric trace species are challenged by the need for sufficient horizontal resolution to identify constituent spatial distribution inhomogeneities (that result from non-uniform sources/sinks and atmospheric transport) and the need for adequate temporal resolution to resolve daytime and diurnal variations. Both of these requirements can be fulfilled from a geostationary Earth orbit (GEO) measurement configuration. An advanced atmospheric remote sensing concept for the measurement of tropospheric ozone from a GEO-based platform is presented. The concept is centered about an imaging Fabry-Perot interferometer (FPI) observing a narrow spectral interval within the strong 9.6 micron ozone infrared band with a spectral resolution approximately 0.07 cm-1. This concept could also simplify other atmospheric chemistry sensor designs (which typically require spectral resolutions in the range of 0.01 - 0.1 cm-1), since such an FPI approach could be implemented for those spectral bands requiring the highest spectral resolution and thus simplify overall design complexity.
Space-based observations of tropospheric trace species have been identified as high-priority atmospheric science measurements to be included in Earth science missions of the 21st century. Critical to such measurements are tropospheric ozone (O3) concentrations, which have been increasing and will continue to do so as levels of the precursor gases (oxides of nitrogen, methane and other hydrocarbons) necessary for the photochemical production of tropospheric O3 remain rising; such a global monitoring capability is crucial to enhance scientific understanding as well as to potentially lessen the ill-health impacts associated with exposure to elevated concentrations in the lower atmosphere. An instrument concept to enable such a measurement capability for tropospheric (and total) O3 utilizing Fabry-Perot interferometry has been developed and reported in earlier work. It involves a double-etalon series configuration Fabry- Perot interferometer (FPI) along with an ultra-narrow bandpass filter to achieve single-order operation with an overall spectral resolution of approximately .068 cm-1, sampling a narrow spectral region within the strong 9.6 micrometer ozone infrared band from a nadir-viewing satellite configuration. Current research efforts are focusing on technology development and demonstration activities to address technology drivers and other design considerations associated with this measurement concept. Most importantly, we have developed a small-scale, modular, double-etalon prototype FPI for laboratory characterization and testing. This presentation will focus on advancements made pertaining to our laboratory prototype, specifically, toward the analysis and interpretation of measured solar absorption spectra. Topics will include processing of 'measured' spectra (i.e., spectral registration and drift correction) and simulation of 'true' spectra (i.e., atmospheric assumptions and instrument transfer function modeling), as well as subsequent comparisons and findings. Future developments will focus on incorporating other key elements into the prototype instrument, performing relevant laboratory and atmospheric testing, and developing methods for calibration. These activities along with concurrent scientific studies and atmospheric field testing will serve to demonstrate overall feasibility and provide technique validation for this instrumentation and may lead to a future space-based implementation.
Sounding the stratosphere using the 4.3 micrometers CO2 lines requires observations where radiance contributions from the very thin upper atmosphere form a significant part of the measured signal. In order to properly account for the influence of these extremely narrow emission features in the temperature retrievals, extremely high spectral resolution is required. The multi-order etalon sounder (MOES) is an instrument which capitalizes on the extremely high spectral resolution achievable with a standard Fabry-Perot etalon and on the regular spacing of the lines in portions of the vibration-rotation spectrum of gases such as CO2. The MOES instrument simultaneously measures several identically shaped CO2 lines when its etalon spacing is set to produce a free spectral range equal to the uniform line intervals in the spectrum. By combining the signals from a large number of lines a MOES sensor greatly improves the signal-to-noise ratio without reducing its inherent spectral resolution. A detailed design for the most recent MOES concept is presented along with its expected/simulated performance parameters.
A sounder using a high-finesse Fabry-Perot etalon offers substantial potential to extract temperature profiles from the stratosphere atmosphere. The multi-order etalon sounder (MOES) is such an instrument. Its extremely high spectral resolution makes it possible to selectively observe emission at the very line centers and near shoulders of individual CO2 lines. However, the radiances at the centers of these lines can contain large non-LTE contributions originating at much higher altitudes. At high altitudes, the non-equilibrium absorption and re-radiation of solar illumination enhances the vibrational temperature compared to the kinetic temperature. The kinetic temperature profile can only be estimated from the complex line radiance spectrum by relating the observed vibrational temperature to the probable kinetic temperature. The effective separation of layer contributions requires that the instrument be designed so that (1) its dynamic range preserves the much larger range of expected radiances and (2) its spectral response function is very well known so that the line wing radiances can be accurately determined in the presence of large line center radiances. This paper discusses the retrieval of stratospheric temperatures in terms of typical measured radiance covariances, the required solar illumination model and a sensor suitable for routine temperature profile retrieval.
Space-based observation of tropospheric pollution has been identified as an important measurement to be included in Earth science missions of the 21st century. This presentation will summarize on-going efforts focused on enabling such a new capability, a high-priority atmospheric science mission for the measurement of tropospheric ozone from a space-based platform, through the implementation of Fabry-Perot interferometry. The measurement technique involves a double-etalon series configuration FPI along with an ultra-narrow bandpass filter to achieve single-order operation with an overall spectral resolution of approximately .068 cm-1, sampling a narrow spectral region within the strong 9.6 micrometers ozone infrared band form a nadir-viewing satellite configuration. Current research efforts are focusing on technology development and demonstration activities to address technology drivers associated with this measurement concept. To this end we have developed a small-scale, modular, double-etalon prototype FPI for laboratory characterization and testing, modelled the instrument optical configuration, and performed R and D associated with an etalon optical control scheme. This presentation will cover advancements pertaining to all aspects of this effort, however, emphasis will be placed on integration and testing activities associated with the laboratory prototype FPI. This will include multichannel operation considerations pertaining to different configurations for spectral tuning. In addition, implications associated with extrapolation toward a full- scale flight instrument design will also be addressed.
Optical remote sensing plays an important role in the study of planetary atmospheres, especially in determining trace gas abundance, temperature profiles and dynamics/winds. Instruments to be flown aboard interplanetary platforms must be small, have low mass and consume little power. Fabry- Perot interferometers (FPI) satisfy these physical constraints and are capable of acquiring spectra suitable for analysis of the atmospheric parameters. Two new applications of FPI technology have recently been developed at UM/SPRL: the multiplex Fabry-Perot interferometer (MFPI) and the multi-order etalon spectrometer (MOES). The MFPI produces a broad bandwidth high resolution spectrum via Fourier transformed interferograms produced by scanning the etalon over large distances. The MOES simultaneously measures several similar lines in a regular spectrum by matching its free spectral range to the line spacing. Thus MFPI provides a means for broadening the usable bandwidth and MOES can record improved signal-to-noise spectra at extremely high resolution. This paper reports recent progress in the design, construction and testing of a prototype instrument incorporating both the MFPI and the MOES concepts using a single set of etalon plates.
In Fourier transform spectroscopy degradation in spectral resolution and the introduction of false spectral features (`feet') both result from the limited optical pathlength used by a real instrument. This work describes a quantitative relationship between the truncation length of an interferogram and the distortion of the computed halfwidth for the three most common spectral shapes: Gaussian, Lorentzian, and Voigt profiles. The technique can also be used to aid in the alignment and calibration of a Fourier transform interferometer by deriving a calculated truncated interferogram lineshape of a `known' spectral line to compare with the lineshape actually produced by the instrument.
Remote sensing of major and minor constituents in the earth's atmosphere is of great importance to the study of climate and global change. Because much of remote sensing involves placing instrumentation in environments that are not easily accessible, such as balloons, spacecraft, or remote field stations, it is usually necessary that the instrumentation be compact, lightweight, and rugged. This paper describes the development of a new type of remote sensing instrument we have chosen to call the multiplex Fabry-Perot interferometer (MFPI). We present atmospheric spectra obtained with our working prototype instrument. The MFPI is a Fabry-Perot interferometer for which the etalon plate separation is changed over a large optical distance during a measurement. When the resulting interferogram is Fourier transformed the multiple reflections within the etalon cavity produce a spectrum analogous to that which would be produced by an array of Michelson interferometers. However, for high spectral resolution measurements the scan distance required by the MFPI is much less than for the comparable Michelson. The MFPI will be ideal for remote sensing applications where weight, size, and mechanical reliability are primary considerations.
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